Polyesters are often prepared by polycondensation of hydroxyacids or dicarboxylic acids and diols, which reaction proceeds at relatively high temperatures. Suppression of side reactions as decarboxylation, dehydration of the alcohol or ether formation can be achieved by transesterification, which reaction is mainly used for the manufacturing of the important polyesters polyethyleneterephthalate (PET) and polybutyleneterephthalate (PBT). Reaction conditions usually involve prolonged heating to temperatures between 240.degree. and 300.degree. C. Less severe reaction conditions can be used when the carboxylic acid group is activated by using the corresponding acylchloride or mixed anhydride. Dehydrating agents as thionylchloride and phosphorous derivatives have been used to polymerize 4-hydroxybenzoic acid (cf. Polyesters 1965, Elsevier, N.Y. 1, 13 and Makromol. Chem. 1981, 182, 681).
A mild method (reaction temperature 100.degree.-150.degree. C.) which also enables the manufacture of compounds with both an alcohol functionality and a carboxylic acid halide in a monomer is disclosed in Makromol. Chem. Rapid Commun. 1980, 1, 457 and consists of protecting the alcohol group with trimethylsilylchloride. This method has been used for the preparation of the poly- and oligomeric ester poly(4-hydroxybenzoate) under mild conditions using 4-(trimethylsilyloxy)benzoylchloride as starting material. Poly(4-hydroxybenzoate), in particular its copolymers are valuable products in connection with their special physical properties useful as a stable engineering plastic (high crystallinity, extremely low solubility and thermotropic liquid-crystallinity) and many efforts have been made to develop a satisfactory manufacturing process with suitable monomers. This does not only apply for ester compositions based on hydroxycarboxylic acid building blocs, but also for ester compositions based on dicarboxylic acid and diol building blocs.
Most of the known ester manufacturing processes outlined above require relatively high reaction temperatures or, alternatively, they require less accessible, expensive, starting materials like silylated phenols, which materials previously need to be prepared from a phenol and trialkylchlorosilane.
Therefore there is a need for an economically attractive industrial bulk manufacturing process using cheap starting materials and operating under attractive economical, environmental and safe conditions, i.e. using rather simple equipment and resulting in a significant reduction of the cost price of the poly- and/or oligomeric ester. Consequently, considerable research and development efforts have been made for an improved manufacturing process for these ester compositions.